The present invention is directed to a choke for suppressing radio interference and is also directed to a method for the manufacture thereof.
In clocked power pack parts, particularly in switched power pack parts, electromagnetic interference occurs due to extremely steep voltage or, current edges during the switching operations of the power pack part. This so-called "broad band radio interference", however, is undesired. The frequency of the radio interference the region of a few hundred kilohertz up into the megahertz range. In conformity with the standards of electromagnetic compatibility (EMC), this radio interference is to be eliminated at the point of creation, (i.e., within the device).
What is probably the most effective method for suppressing radio interference is the employment of what are referred to as single-conductor chokes. Single-conductor chokes are chokes for suppressing radio interference that are fashioned as annular magnetic tape cores that can be plugged onto a wire or onto a terminal pin of a circuit component. Such chokes for suppressing radio interference are known, for example, from the Data Book of the Toshiba Corporation, Material & Components, Technical Data, "Amorphous Noise Suppressor, AMOBEAD.TM., Serial No. E-63001, Jan. 30, 1988.
Compared to other components for suppressing radio interference, such as a RC low-pass, for example, single-conductor chokes have the advantage of high inductances even given high choke currents as well as a broad band interference suppression effect in the range from 10 kHz through 30 MHZ. Further, they also exhibit an especially high insertion attenuation in the lower frequency range. Finally, they exhibit low overall losses and low structural sizes.
What are referred to as single-conductor chokes in the form of small, wound magnetic tape cores of amorphous alloys, particularly on a cobalt basis, are discussed in the aforementioned document. The wound magnetic tape cores are slipped or, plugged onto the conductors carrying the current in the component part causing the interference. There, the chokes act as saturatable chokes with whose assistance high-frequency disturbances can be effectively combated during a switching event. Due to the saturation of the magnetic material of the magnetic tape core, however, influencing of the circuit to be protected no longer occurs following the switching event.
In the manufacture of such a magnetic tape core composed of an amorphous alloy, however, the tape to be wound is usually secured to a winding shaft made of tool steel with a spot weld. After the welding, the magnetic tape core is wound to desired geometrical configuration. Finally, the tape end is, in turn, secured to the outside circumference of the magnetic tape core with a spot weld. After the end of the welding process, the magnetic tape core is shorn from the winding shaft. The annular magnetic tape core that has been formed as a result can then be further-processed in a known way. In particular, the magnetic tape core is subjected to a thermal treatment and is subsequently covered with a passivation layer.
Such single-conductor chokes, however, are complicated to manufacture since the annular component parts must be manually attached over the terminal pins, for example of a transistor or a diode. The adjustment of the annular single-conductor choke around the terminal pins thereby plays a particularly large part and requires additional assembly outlay.
Another critical disadvantage derives from the extremely poor thermal contact of the magnetic tape core relative to the terminal pins of the circuit and an inadequate elimination of the unwanted heat from the magnetic tape core that is caused as a result thereof The dissipated heat arising, for example, given a magnetization into saturation at frequencies in the range of a few hundred kilohertz thus usually leads to a heating of the components of more than 100.degree. C. As a result of these high temperatures and due to the magnetic field in winding direction generated by the operating current, a tempering occurs that, unbeneficially causes a rectangular hysteresis loop that in turn, intensifies the magnetic field in winding direction. Over time, however, the alloys that are utilized cannot overcome these high temperatures, this leading to an aging of the material of the magnetic tape core with corresponding changes in the magnetic properties of the alloys. These usually cause a further increase in the re-magnetization losses, which can ultimately lead to the thermal failure of the choke.
European reference EP A 0655754 discloses an inductive component, whereby a magnetic foil is tightly wound around a terminal wire to form a magnetic tape core.